This invention relates to ultraviolet/visible/infrared spectroscopy (UV-VIS) and in particular to a spectrophotometer therefor which offers variable spectral resolution (variable slit bandwidth) in combination with the use of a pulsed light source, for example a xenon flash tube, and solid state detectors. The invention will be described in relation to the use of a xenon flash tube, but it is to be understood other pulsed light sources having similar characteristics may be used.
Solid state photosensitive devices (typically silicon) have several advantages as optical detectors for UV-VIS over the more traditional photomultiplier (PM) tubes. Considering specifically the case of silicon
the useable wavelength range is 190-1100 nm compared to 190-900 nm for photomultiplier (PM) tubes.
the conversion efficiency of photons to free electrons (photosensitivity) is higher for silicon than for PM tubes and may be, for example, as high as 600 milliamps per watt compared to photomultiplier efficiencies typically peaking at 40 milliamps per watt. This leads to lower system noise levels.
Silicon detectors exhibit very uniform photosensitivity over their surface. By contrast the photosensitivity of PM tubes varies significantly at different points on the photosensitive cathode.
Silicon detectors are small and robust. By contrast PM tubes are large and fragile.
The photosensitive surface of silicon detectors is flat and fully accessible. By contrast the photosensitive surface of PM tubes is located some distance inside an evacuated glass envelope.
Silicon detectors are cheaper than PM tubes and require less electronics to operate.
For the above reasons, replacement of PM tubes with silicon detectors in UV-VIS instruments is highly desirable and indeed has occurred in lower priced instruments.
It should be noted that solid state detectors are not limited to silicon. Germanium, Gallium Arsenide, Lead Sulphide and many other solid state detectors also exist and much of the above also applies to these devices. These alternative detectors typically cover other wavelength ranges. None the less, silicon is the most commonly used and this invention will be described in terms of this detector type, however it is to be understood that the invention is not limited to the use of a silicon solid state detector.
Xenon flash lamps also have many advantages over conventional continuous emission light sources.
Optical emission covers the entire wavelength range from below 190 nm to above 1100 nm eliminating the need for two or more light sources plus the attendant optical switching means.
The xenon light source is far more efficient at converting electrical power into light allowing a significant reduction in power input and heat generation. For example, a quartz halogen light plus deuterium arc lamp combination typically used in conventional UV-VIS instruments together with power supplies typically consumes up to 120 watts of electrical power. Essentially all of this ends up as heat which has to be removed from the instrument. By contrast a xenon flash lamp typically consumes less than 10 watts peak and appropriate system design techniques can reduce the average power consumption even lower (to below 1 watt). Not only does this reduce heat generation, it also makes portable or battery operation feasible.
Since the xenon flash lamp emits light in a discontinuous fashion compared to the continuous emission of conventional sources it eliminates the need for optical interrupters or choppers simplifying the mechanical construction of the instrument.
Because the xenon emission is in the form of a very short (typically less than 10 microseconds) and very intense light pulse it allows room light compensation to be made very close in time to the light measurement thus improving the accuracy of such compensation and rendering the instrument substantially less sensitive to room light ingress (typically up to 10,000 times less sensitive).
Xenon flash lamps have substantially longer life than conventional sources. Depending on the system design, this can be up to 40-80 times as long which substantially adds to overall instrument reliability.
However xenon flash light sources have a substantial disadvantage over conventional light sources. This is that the position and intensity of the arc varies from flash to flash, which has the effect of substantially varying the received optical intensity flash to flash. This variation is effectively optical noise and one technique for overcoming this noise, disclosed in International Application No. PCT/AU97/00603, involves splitting the optical beam into two parts, passing one part through the sample (giving a sample beam), bypassing the sample with the other part (giving a reference beam), simultaneously measuring both the sample and reference beams and ratioing them.
Another problem with xenon flash lamps is that the spatial intensity distribution within the beam also varies. To avoid the effects of this, all parts of the incident beam need to be uniformly split into the sample and reference beams (for example, a split of the left half of the incident beam to the sample and the right half to reference would be unacceptable). It is also necessary that the detectors which are used have uniform photosensitivity over their optical surface. This need for uniform photosensitivity can be met by using solid state (silicon) detectors in place of PM tubes.
The advantages to be gained by the combination of a pulsed light source such as a xenon flash lamp and a solid state detector have been realised by the recent marketing of such an instrument by the applicant. This instrument was introduced to the market as the xe2x80x9cCary 50xe2x80x9d in October 1997. Dynamic range problems in this instrument due to variations in light emission, optical throughput and detector sensitivity and spectral bandwidth with wavelength are able to be handled by the use of a fixed slit (i.e. 1.5 nm width) and appropriate selection of optical system components. However the desire to provide a variable slit instrument which retains the advantages of a pulsed light source and solid state detectors presents a dynamic range problem of far greater magnitude than that encountered in the known Varian (fixed slit) instrument. This is because the energy throughput of the optical system is proportional to the square of the slit bandwidth. Thus for a system with slit settings ranging from 0.25 nm to 4 nm using a broadband grating, and for wavelengths from 190 nm to 1100 nm, a dynamic range of about 77000:1 (before sample absorbance is considered) would be required of the detector. For solid state detectors, such a range in light levels cannot be handled by electronic signal processing gain changes whilst maintaining acceptable noise performance (unlike in PM tubes which allow essentially noise free amplification between the photocathode and the anode, the degree of which is controllable by the voltage level applied to the tube). Generally, UV-VIS instruments which offer variable spectral resolution (i.e. variable slit bandwidth) must be able to cope with a much larger dynamic range of light levels compared to fixed slit instruments. Typically, this range is 200-400 times larger.
An object of the present invention is to provide a UV-VIS instrument which utilizes a pulsed light source and solid state detectors and which allows variable spectral resolution.
In a first aspect the invention is characterised by varying the light energy emitted by the pulsed light source on a flash by flash basis. According to this aspect there is provided a spectrophotometer for UV-VIS spectroscopy including:
a pulsed light source for emitting bursts of light,
an optical system for directing a beam of each said burst of light to a sample to be analysed, the optical system including a monochromator for selecting the wavelength of said beam, and having a variable slit size,
a solid state detector for detecting the intensity of light after interaction of said beam with said sample,
and a variable source of electrical power connected to the pulsed light source for varying the energy of each burst of light emitted by the light source for controlling the dynamic measurement range of the spectrophotometer.
Preferably the pulsed light source is a xenon based flash lamp.
Thus the light energy of the source per flash (that is, per burst of light from the lamp) can be manipulated to control the dynamic measurement range of the spectrophotometer in the sense that the actual dynamic range that is presented to the detector for measurement is reduced.
Preferably the light energy is varied in dependence on either or both the wavelength or slit settings which are chosen for a measurement.
This aspect of the invention also provides a method of conducting spectroscopic analysis of a sample including the steps of:
generating bursts of light by a pulsed light source,
directing a light beam generated by each burst through the entrance slit of a variable slit bandwidth monochromator to the sample to be analysed,
measuring the intensity of said light beam after it has interacted with said sample using a solid state detector, and
varying the energy of each burst of light by the pulsed light source to control the dynamic measurement range at the detector.
Preferably the pulsed light source is a xenon based flash lamp.
The light energy emitted by a xenon lamp depends on the electrical power input which is usually derived from charge stored on a capacitor. This charge is dumped into the lamp to generate a flash by a trigger pulse. The charge stored on a capacitor depends on the capacity of the capacitor (C) and the voltage (V) applied thereto and the invention encompasses changing either C or V or both to vary the light energy emitted by the xenon lamp. There are several ways in which the capacitance voltage may be changed. For example, one possibility is to vary the time for which the capacitor is charged from either a fixed current source or a fixed voltage source and series resistor. In the preferred embodiment the voltage is determined via a feedback loop setting the voltage in relation to a voltage reference output from a processor. This establishes more precise control of voltage. Also in the preferred embodiment the feedback loop operates through controlling the output of a DC:DC converter. Lower voltages will in general result in shorter charging times. This allows for increasing the flash rate as the energy per flash is reduced without requiring a larger charging supply or increasing the average dissipation of the lamp. Such an arrangement has the benefit of using regions of greater light throughput to increase data collection rate or reduce noise. It has the disadvantage of increasing the complexity of data collection timing.
A second aspect of the invention is to vary the slit height when changing width. Thus, according to this second aspect, there is provided a spectrophotometer for UV-VIS spectroscopy including:
a pulsed light source for emitting bursts of light,
an optical system for directing a beam of each said burst of light to a sample to be analysed, the optical system including a monochromator for selecting the wavelength of said beam, and having a variable slit size,
a solid state detector for detecting the intensity of light after interaction of said beam with said sample,
and a source of electrical power connected to the pulsed light source for providing electrical energy for each burst of light emitted by the light source,
wherein the slit size is variable by varying both the slit width and the slit height such that as width is increased, height is reduced, for controlling the dynamic measurement range of the spectrophotometer. For example, an 8 mm high slit offering 0.5 nm spectral bandpass will have about the same light throughput as a 2 mm high slit offering 1 nm bandpass.
Preferably the pulsed light source is a xenon based flash lamp.
The two aspects of the invention described above may each be used in isolation or jointly to reduce the dynamic range presented to the detector and electronic system.
Other features of the invention are as defined in the claims.
An embodiment of the invention will now be described, by way of non-limiting example only, with reference to the accompanying drawing.